Urination out of litter box

My sister in law moved in with us about 3 months ago with her domestic long hair.  He's been peeing outside of the litter box and I was wondering if anyone knew what to do to prevent this.  He's about 5-6 years old and I would guess he's fully in tact though she refuses to get him neutered(despite me being adamant that it would solve 99% of the issues).  He's peed on floors, shoes, cat toys, and has even peed on some of our clothing in our closet including my wife's wedding dress.  We now keep our bedroom door closed due to his peeing.  Despite the change in environment I don't think that's 100% of the issue because according to my sister and the smell in her old apartment he has been doing this for years.  It was so bad that he was having seizures from the ammonia smell in her old apartment*.

Its gotten to a point where the other cats are starting to mark the spots where he has peed despite our best efforts to clean them.  My oldest dsh, Tomnus, even peed in my shoes the other day just after I took them off when I came home from work.  He then proceeded to run over to me and wrap himself around my legs while purring.  I'm sure he was trying to tell all the others that I'm his but I didn't really appreciate the sentiment at the time.  Tom has been fixed which is another reason why this troubles me because I didn't think fixed cats ever went outside of the box.  I'm sure his doing it, like I said earlier, is to show the others that I'm his though and to regain some semblance of control in his environment.

Any suggestions on how to keep my sisters cat from peeing outside of the littler box?  It should be mentioned that he will also use the box as well.

The behavioral question is  - are there enough litter boxes? Theory is you need one litter box per cat plus one and repeated on each floor. So I have a two story townhouse and 3 cats so 8 litter boxes total. 

I'm sure the other cats are marking because of this new stranger. Do they get along besides the improper urination?

And not to frighten you but one of my cats would use the litter box and mark outside the box. We ran as many tests as we could for over a year. When I finally switched vets she was determined to locate the cause and did an ultrasound which found three large bladder stones. 

Has your sister had her cat ultrasound recently to determine he is not suffering from anything internally?

She's never had him to the vet actually.  We have enough box's and the cats get along okay.  Every once in a while they get a hair up their butts and chase each other around the house but no blood is ever drawn or anything.  Since he's moved in we've gotten 2 kittens and we haven't noticed any issues with the kittens but I'm sure they'll start following the example of the older cats before to long.

I'd definitely take him to the vet as there could be an underlying medical issue. 

He needs to be neutered!  This is what un-neutered cats do.. you know that, but it sounds like your sister just won't buy it.

I honestly don't know how you are putting up with it as it's only going to get worse and as you've seen, it's going to really affect your cats.

Get the cat checked out by a vet and get him neutered. Those should always be the first 2 go-to options when cats show behavioral problems.

She doesn't want to get the cat neutered? Well, she's a guest in your home, she has to abide by your rules and demands.

A 5 year old cat that's never been to the vet?  That would be #1 on my list of things to do.

Um, yeah, if someone (and their cat) were staying in my house, I'd have some rules about how their cat needed to be kept :/. Insist on him being neutered and fully vetted. The problems aren't going to get any better if not. Plus if he's unvaccinated he could be a threat to the kttens as their immune systems aren't well developed. He could also be a threat to the kittens in another way, as he'll try to mate with a female and try to chase any junior males away. So, yeah, that's just what needs to be done. Any other suggestions aren't going to help.

did your cat consistently urinate out of the litter box or just occasionally? My mature cat, after a uti cleared up, still pees outside of box but not all the time.